Multi-stage infectious waste treatment system

ABSTRACT

A multi-stage treatment system for infectious waste includes a shredding stage, a granulating stage, a wetting stage, a disinfecting stage, and a dewatering stage which define a continuous treatment flowpath for the infectious waste. A plurality of blades shred the waste in the shredding stage, then the waste is injected with volatile disinfectant chemicals which are mixed immediately before injection. A plurality of blades in a granulating stage then fragment the waste to a smaller particle size. The granulating stage insures that the waste is granulated to a sufficiently small size to facilitate the use of a relatively low concentration of a highly reactive disinfectant. Chemicals are mixed to form a volatile, highly reactive disinfectant which is then immediately injected into the waste downstream of the shredding stage. A plurality of jets wet the waste mixture in the wetting stage with the heated aqueous disinfectant. A flow restriction removes excess aqueous liquid from the disinfected waste in the dewatering stage and renders the product suitable for landfilling. A control system controls the temperature of the disinfectant to maintain an optimum temperature for a desired kill rate.

RELATED APPLICATIONS

This file is a continuation-in-part application of prior patentapplication for a "Multi-Stage Infectious Waste Treatment System", Ser.No. 214,597, filed Mar. 18, 1994 now U.S. Pat. No. 5,425,925, which is acontinuation-in-part of an application for a "Method for Sterilizing andDisposing of Infectious Waste," Ser. No. 690,116, filed Apr. 23, 1991now abandoned, which was a continuation-in-part of application Ser. No.511,275, filed Apr. 19, 1990, now U.S. Pat. No. 5,089,228.

FIELD OF THE INVENTION

The present invention relates generally to treatment of infectiouswaste. More particularly, the present invention relates to a systemwhich mechanically fragments and decontaminates infectious waste. Thepresent invention is particularly, though not exclusively, useful fortreating an infectious waste stream which includes a variety of types ofwaste.

BACKGROUND OF THE INVENTION

The disposal of infectious waste from hospitals and other medicalestablishments is a major problem. Indeed, the importance of proper andeffective infectious waste disposal has become of greater concern inrecent years, due to an increased awareness of health problems such asthe AIDS epidemic. In part because of the AIDS epidemic, definitions ofwhat constitutes "infectious waste" are being broadened. Consequently,the volume of infectious waste which must be disposed of is increasing.Accordingly, the need for a system or apparatus which will accomplishthe safe, efficacious, and cost effective treatment of significantvolumes of infectious waste for disposal is growing.

One method for decontaminating infectious waste involves incineration,wherein the waste is burned and the decontaminated ashes are properlydisposed. An alternative treatment method is to disinfect the waste in asteam autoclave prior to waste disposal. While effective for theirintended purposes, both incinerators and autoclaves present ancillaryproblems. Incinerators, for example, are difficult and costly toconstruct and are relatively expensive to maintain in an environmentallysafe manner. Autoclaves too, present additional problems, such as odor,cost and operational complexity. Additionally, waste which has beendisinfected by autoclaving typically requires further treatmentprocedures, such as incineration or shredding and granulation, prior tofinal disposition of the waste in such places as landfills.

With the above discussion in mind, alternative infectious wastetreatment systems have been proposed to disinfect the waste inpreparation for disposal. According to these proposals, a solidinfectious waste is contacted with a disinfectant solution containing achlorine compound to decontaminate the waste. The decontaminated wastemay then be disposed in ordinary landfills.

Unfortunately, decontamination of waste using chlorine compoundspresents certain technical complications. First, liquid disinfectantloses its disinfectant potency during prolonged storage. Thus, there isa need to use liquid disinfectant that is relatively "fresh" in order toachieve an acceptable degree of waste decontamination. Second, it isrelatively difficult to ensure that an appropriate concentration of thedisinfectant has contacted the waste during the treatment process. It isalso important, however, to avoid applying too high a concentration ofchlorine compound to the waste, in order to avoid undesirable results,such as corrosive effects and the release of toxic gases. Significanthealth risks are known to result from the discharge of chlorine to theenvironment.

The most commonly used disinfectant is sodium hypochlorite, typically asa one percent solution. The strength of the solution is dictated by thenecessity of achieving a desired rate of bacteria kill in a givenapparatus, resulting in a given rate of use of the disinfectant whenoperating at a given rate of throughput of waste. The use of a onepercent solution results in the discharge of a significant amount ofchlorine into the environment from the typical apparatus, either intothe sewer or absorbed into the processed waste.

Because of its higher reactivity, chlorine dioxide is far more effectivethan sodium hypochlorite for the treatment of infectious waste. Chlorinedioxide also typically exists as a gas in solution, greatly enhancingthe penetration of the disinfectant into the waste material. Chlorinedioxide can, if applied to properly granulated waste, achieve thenecessary kill rate at a concentration of only about 50 ppm, or onlyabout 0.005 percent, or 5 one-thousandths of the necessary concentrationof sodium hypochlorite. Finally, the chlorine dioxide is far lessenvironmentally persistent, rapidly disassociating into sodium chloride,water, and citric acid. When taking into account the rate of use ofsodium hypochlorite in a typical process, and the required rate of useof properly applied chlorine dioxide to achieve the same kill rate, thesodium hypochlorite process results in the discharge to the environmentof approximately ten thousand times as much of the treatment chemical.This means that the chlorine dioxide process results in the discharge tothe environment of an amount of chlorine which is minuscule, compared tothe amount of chlorine discharged by the sodium hypochlorite process.

Unfortunately, chlorine dioxide is very corrosive, highly unstable, andeven explosive. It can not simply be substituted for sodium hypochloritein a process. It must be used in an apparatus designed to properlygenerate and mix the chemical, and designed to properly granulate andhandle the waste material to allow the use of a very low concentrationof the chemical. Further, it is impractical to store chlorine dioxide,because it is an oxidizer and exposure to oxidizable impurities wouldconsume a portion of the chlorine dioxide. Effective exclusion ofoxidizable impurities from the storage tank would be difficult. Becauseof these difficulties, sodium hypochlorite is almost always usedinstead, even though it is less effective, and even though it results inincreased chlorine contamination of the environment.

The present invention recognizes that liquid precursors of chlorinedioxide can be stored for relatively lengthy time periods without losingtheir potency. Further, these liquid precursors can be mixed to formchlorine dioxide immediately prior to injection into a waste stream, ina continuous process. Chlorine dioxide can be produced by thecombination of sodium chlorite with a strong mineral acid, such ashydrochloric acid, or with a weak organic acid, such as citric acid. Theuse of a strong acid tends to produce high concentrations of chlorinedioxide more rapidly, and the process tends to be more difficult tocontrol. Treatment of medical waste requires very low concentrations ofchlorine dioxide produced in a slow and predictable manner. Therefore,the best choice for treatment of medical waste is the combination ofsodium chlorite with a weak organic acid, such as citric acid. Theresulting reaction is slow and predictable, and it is easily mediated bycontrolling the temperature of the solution.

The resulting solution can be used in a very low concentration todecontaminate infectious waste, if used in a system that mechanicallyreduces the particle size of the waste to the appropriate size. Thepresent invention also recognizes the necessity for the correctinteraction of certain critical structural features in the wasteprocessing apparatus, to achieve the necessary intimate contact betweenthe low concentration of chlorine dioxide and the waste material, and toproperly handle the waste material to allow the conservative use of thechlorine dioxide.

Accordingly, it is an object of the present invention to provide asystem for waste treatment in which chlorine dioxide precursors areappropriately mixed and then immediately blended with infectious wasteto decontaminate the waste, while preventing excessive decomposition ofthe disinfectant, and while preventing any explosion hazard. Anotherobject of the present invention is to provide a system for wastetreatment which results in the reduction of waste particle size to anappropriate size to allow effective use of the disinfectant in a lowconcentration, while preventing clogging of the waste stream and whilemaximizing the recycling of the disinfectant. Finally, it is an objectof the present invention to provide a system for waste treatment whichis relatively easy and comparatively cost-effective to implement.

SUMMARY OF THE INVENTION

The present invention is a system for treating infectious wastecomprising a series of continuous treatment stages. The multi-stagetreatment system has an inlet stage at its front end which comprises anopening for receiving the infectious waste. The waste may be fed in anyform through the opening, but in a preferred embodiment, the opening issized to receive a sealed plastic bag in which the waste is packaged.The bags are fed through the opening into the system in their entirety.In this manner, waste handlers operating the present system need nevercome in direct contact with the infectious waste. The waste bag has aprimary compartment containing the infectious waste, and it can have oneor more secondary prefilled and sealed compartments containing otherprocess additives in isolation from the waste, all of which are to beintroduced into the system. As will be seen below, the entire contentsof the bag are released from the bag and commingled during operation ofthe treatment system.

The inlet opening leads to a fragmenting chamber positioned therebelow,which encloses the shredding, wetting, and granulating stages of thesystem. The waste drops under the force of gravity from the inlet stagedown into the shredding stage which comprises a plurality of opposinglyrotating shredder blades. The shredding blades destroy the waste bag,spilling its contents into the fragmenting chamber. Any processadditives contained in the bag become mixed with the waste in theshredding stage. The blades also function to break up any largefrangible waste into small size particles.

The wetting stage is positioned immediately beneath the shredding stageto wet the small particle size shredded waste with the liquid chlorinedioxide treatment solution, as the waste falls through the shreddingblades. The liquid disinfectant will more thoroughly mix with the wastematerial as the waste passes farther through the system. The wettingstage comprises a plurality of jets through which the liquiddisinfectant is pumped, with the jets being positioned at the interiorwalls of the fragmenting chamber immediately beneath the shreddingblades. The jets are directed radially into the chamber and are capableof producing a controlled spray of the liquid disinfectant into thewaste mixture. The liquid disinfectant is preferably an aqueous chlorinedioxide solution containing some gaseous chlorine dioxide, which hasbeen formed by the continuous mixing of liquid sodium chlorite andcitric acid immediately prior to injection through the jets. The liquiddisinfectant is also maintained at an elevated temperature immediatelyprior to injection. The temperature of the disinfectant is controlledaccording to the measured chlorine dioxide concentration in gas strippedfrom the solution. The heated liquid is then injected, to uniformlycontact the falling waste mixture to form a hot mash. While the presentinvention in its preferred embodiment makes possible the use of chlorinedioxide as the disinfectant, the apparatus can be used with otherdisinfectants without departing from the spirit of the invention.

The granulating stage is provided beneath the wetting stage andcomprises a plurality of specially designed blades mounted on a shaft soas to be rotatable against a plurality of stationary blades mounted onthe walls of the fragmenting chamber to form cutting surfaces. Thegranulating blades rotate in a radial plane which is substantiallyparallel to the flow of the mash. At the cutting surfaces, thegranulating blades break up the already small particle size waste intoyet smaller particle sizes, to insure intimate contact between thetreatment chemical and the waste material, and to cut any fibrousmaterial which has not been previously fragmented by the shreddingblades. The granulator blades are designed to allow the use of aplurality of cutting edges as the edges wear as a result of contact withhard materials. The blades also fully mix the components of the wastemash, thereby ensuring that the disinfectant chemicals in the liquidmedium adequately contact the waste material to achieve the necessarykill rate with a relatively low concentration of the chemical.

A granulator is used instead of a hammer mill, because a hammer millwill not consistently and efficiently reduce the particle size ofnon-brittle materials, which constitute the majority of the medicalwaste stream. When such soft materials are passed through a hammer mill,the materials tend to pass through in crumpled form, rather than havinga reduced particle size. This results in reduced contact between thedisinfectant and the waste material.

A granulator is used at this stage instead of a shredder, because ashredder produces long strips of material. The strips tend to clog theshredder if used with a sizing screen, and they tend to become folded inaccordion folds, thereby reducing contact between the disinfectant andthe waste material. A shredder can also pass relatively large itemsunscathed. If a sizing screen were used downstream of a shredder at thispoint, it would quickly clog.

The output of the granulating stage is preferably fully wetted by thedisinfectant solution, and it has a smaller granular particle size thanthe output of the shredding stage. The granulating stage is alsodesigned with a sizing screen interacting with the specially designedblades to insure that the waste material is repeatedly cut andseparated. The blades tend to press outwardly on the waste material inaddition to cutting it, partially forcing the waste material through thescreen. The blades also drag waste material cross the surface of thescreen, thereby dispersing any tightly packed clumps of the material.This insures that the waste material does not clog the screen, and thatit is reduced to the appropriate size to achieve thorough contact withthe disinfectant, thereby allowing use of the desired low concentrationof chemical.

The outlet from the fragmenting chamber incorporates the aforementionedscreen functioning in cooperation with the granulating blades. Thescreen is sized to allow a selected smaller granular particle size wasteto fall through the screen into a disinfectant reactor chamber below,while retaining any waste which has not been sufficiently granulated inthe granulating stage. Waste which is retained by the screen is scoopedup by the granulating blades rotating against the screen and returned tothe associated cutting surfaces for additional particle size reductionuntil the waste is sufficiently small to pass through the screen. Up tothis point substantially all of the work to convey the waste through theabove-recited stages is performed by gravity.

The granulating stage is followed by the disinfecting stage. Thedisinfecting stage comprises a disinfectant reaction chamber preferablyintegral with an auger. The auger has two ends; a liquid mediumcollection tank and inlet port are at one end of the auger and adisinfected solid waste discharge port is at the other end. The auger isinclined upwardly to convey the waste from the inlet port to thedisinfected solid waste discharge port. The length of the auger whereinthe disinfection reaction occurs constitutes the disinfectant reactionchamber. The disinfection reaction is preferably completed by the timethe waste reaches a point about two-thirds up the auger incline. Thecontrolled rate at which the auger screw carries the waste up theincline to the discharge port enables a sufficient residence time fordisinfection of the waste.

The disinfecting stage is combined with a dewatering stage. Thedewatering stage comprises a conical flow restriction immediately priorto the discharge port. Although some of the liquid medium is removedfrom the waste by gravity at the lower end of the auger, the bulk of theliquid medium is removed from the waste by compressing the mash throughthe flow restriction. The final exit from the auger is positioned at orslightly beyond the point at which the conical flow restriction begins.The conical flow restriction is constructed with a critical restrictionangle best suited to dewater the waste being treated. When thesefeatures are combined with the proper granulating of the waste, and whenthe waste is free of long strips, a high degree of dewatering willresult without resulting in clogging of the discharge path. The auger isalso sloped at a critical angle best suited to assist in dewateringwhile avoiding clogging. The combination of the pressure rise in themash resulting from the conical restriction, and the pressure riseresulting from the angle of the auger, yield a compression of the wastematerial which achieves the maximum dewatering efficiency withoutresulting in clogging.

The liquid medium driven from the waste mash exits the auger throughperforations or a screen in the housing surrounding the auger, and theliquid is collected and passed to the heated disinfectant mixing tankfor recycling to the wetting stage. The screen in the auger housing isshaped to conform to the radial edges of the auger, so that as the augerturns it continually scrapes compacted waste material from the screen.This prevents clogging of the screen. The liquid to be recycled ispassed through a cyclone separator designed to remove heavy fines orother material prior to return of the liquid to the jets. The heavyfines or other material can be periodically dumped from the cycloneseparator onto the waste material in the auger.

In operation, process control for the present system is provided byregulating the disinfectant concentration in the system and the liquidmedium temperature. Temperature is related to the rate of liquid mediumflow, and heater and auger operating parameters. The above-describedsystem satisfies the present objective of providing an infectious wastetreatment apparatus which contacts an infectious waste with preciseamounts of a disinfectant to disinfect the waste while simultaneouslyfragmenting the waste to reduce its bulk volume. The system alsoprovides an infectious waste disposal apparatus which is relatively easyand comparatively cost-effective to implement and operate.

The novel features of this invention, as well as the invention itself,both as to its structure and its operation, will be best understood fromthe accompanying drawings, taken in conjunction with the accompanyingdescription, in which similar reference characters refer to similarparts, and in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the multi-stage waste treatmentapparatus of the present invention;

FIG. 2 is a view of the shredding stage of the apparatus of FIG. 1,along line 2--2;

FIG. 3 is a schematic view of the apparatus of FIG. 1;

FIG. 4 is a section view of an alternate embodiment of the waste outletflow restriction;

FIG. 5 is a schematic of a control unit for the apparatus of the presentinvention;

FIG. 6 is a generalized curve for the functional relation betweendisinfectant solution temperature and disinfectant concentration;

FIG. 7 is a schematic view of the granulating stage of the apparatus ofFIG. 1;

FIG. 8 is an enlarged view of a portion of FIG. 7, showing therelationship between the blades in the granulating stage;

FIG. 9 is a schematic view of the auger of FIG. 1, with a cycloneseparator; and

FIG. 10 is a graph showing an operating range of desired disinfectanttemperature versus disinfectant concentration.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring initially to FIGS. 1 and 3, the infectious waste treatmentsystem of the present invention is generally designated 10. System 10comprises a plurality of treatment stages including an inlet stage 12, ashredding stage 14, a wetting stage 16, a granulating stage 18, adisinfecting stage 22, and a dewatering stage 24, which define acontinuous flowpath for the waste. The terms "disinfect" and"decontaminate" are used synonymously herein and refer to thedestruction of a substantial portion of infectious constituents withinthe infectious waste sufficient to render the waste substantiallynoninfectious.

Inlet stage 12 comprises an opening 28 at or near the top of afragmenting chamber 30 which houses stages 14, 16, and 18. Inlet stage12 opens down into shredding stage 14 at the upper level of fragmentingchamber 30. As shown in FIG. 2, shredding stage 14 comprises multiplepairs of rotatable shredding blades 31a,b, 32a,b, 33a,b. Blades 31a,32a, 33a, are mounted on shaft 34 and blades 31b, 32b, 33b are mountedon shaft 35 such that blade 31b is rotatably fitted between blades 31a,32a and so on for all the blades as shown. Each rotatable shreddingblade is a disk 36 having a plurality of hook-shaped teeth 37 about theperiphery 38 of disk 36. Stationary shredding blades such as 39a, 39bare fixed to chamber walls 40 spaced appropriately from rotatable blades31a, 32a, 33a and 31b, 32b, 33b to channel waste into the rotatableblades, to reduce the waste particle size to a first selected particlesize, and to prevent waste from accumulating in shredding stage 14. Theoutput of waste material from the shredding stage 14 will include somelong strips of relatively soft material. Shafts 34, 35 are positionedhorizontally and parallel to one another, and the rotatable bladesrotate in vertical planes which are substantially parallel to thevertical flowpath of the waste. Shredding action is provided by rotatingshaft 34 in the opposite direction from shaft 35.

Referring to FIG. 3, a wetting stage 16 is provided immediatelydownstream from shredding stage 14. Wetting stage 16 comprises aplurality of liquid disinfectant jets 42a,b,c,d which are mounted in thewall 40 of chamber 30 around the periphery of the waste flowpath andadjacent the bottom side of the shredding blades. A liquid medium feedline is connected to each jet. Thus, as shown in FIG. 1, liquid mediumfeed lines 46a,b,c,d are connected to jets 42a,b,c,d respectively. Feedlines 46a,b,c, are also connected to a recycle pump 48 across a liquiddistribution manifold 50. Pump 48 receives liquid medium from a recycleline 54 connected to a liquid medium collection tank 56 of a recyclestage 26. The manifold 50 is also fed by the output of a disinfectantmixing tank 156. Precursors, or constituents, of the desireddisinfectant are fed into the mixing tank 156 through inlet lines 158.The disinfectant thus formulated is then immediately pumped from themixing tank 156 into the manifold 50 by pump 160 and disinfectant line162. This arrangement is particularly useful where the desireddisinfectant is very volatile, such as chlorine dioxide. The liquidprecursors can be sodium chlorite and citric acid. Recycled disinfectantcan be fed back into the mixing tank through a recycle line 144. Recycleline 144 can be fed by a cyclone separator as will be discussed later.The mixing tank 156 can be maintained at a selected elevated temperatureby heaters in the tank.

If it is desired to remove metals from the waste stream after shreddingand wetting, a metal segregating stage 58 may be provided immediatelyafter stages 14, 16. Metal segregating stage 58 comprises a magnet 60which is mounted in the wall 40 of chamber 30. Magnet 60 contacts thewaste as it falls toward granulating stage 18 to segregate the metalstherefrom. Access is provided in wall 40 to enable periodic removal ofmetals from magnet 60.

Granulating stage 18 is positioned at the lower level of fragmentingchamber 30 and comprises a plurality of rotatable granulating blades62a,b,c,d,e and stationary granulating blades 64a,b. Referring to FIGS.3, 7, and 8, the rotatable blades 62a,b,c,d,e are mounted on a rotatingshaft 66 which in turn is rotatably mounted on chamber wall 40. Therotatable blades have a vertical plane of rotation which issubstantially parallel to the vertical flowpath of the waste. Therotatable blades 62a,b,c,d,e are rotatable past stationary granulatingblades 64a, 64b, each of which is fixably mounted on opposite sides ofchamber wall 40 adjacent rotatable blades 62a,b,c,d,e. As rotatableblades 62a,b,c,d,e rotate, they periodically pass stationary blades64a,b to form transient cutting surfaces. FIG. 3 shows rotatable blade62e meeting stationary blade 64b to form transient cutting surface 68.The rotatable and stationary granulating blades are all preferablyformed with rectangular cross-sections as shown, so that each blade hasfour potential cutting edges. FIG. 8 shows the relationship between thecutting edges in more detail. Each blade can be removed and rotated toexpose a new cutting edge, until all four edges on each blade have beendulled. As each rotatable blade passes each stationary blade, theclearance between the cutting edges is sufficiently small to granulatethe material by a cutting action. The material is then continuallypassed through the blades until sufficient cuts have been made to reducethe waste material to a selected second, smaller particle size. Inaddition, all strips of waste material are granulated by this cuttingaction.

A screening action is accomplished immediately beneath the granulatingblades in granulating stage 18. This comprises a screen 70 stretchedcross-sectionally across conduit 72 which connects fragmenting chamber30 and auger 74. Screen 70 has a mesh size which allows particles at orbelow a given particle size to pass through while preventing particleshaving a larger particle size than the given particle size from passingthrough. The movement of the rotatable blades imparts an outward radialmotion to the waste material which partially imbeds the waste materialin the screen 70. Screen 70 preferably has a 1/2 inch mesh size althoughother mesh sizes are within the purview of the skilled artisan. Screen70 is positioned to cooperate with the rotatable granulating blades62a,b,c,d,e of granulating stage 18. As the rotatable blades rotate,they periodically pass screen 70 to scoop waste retained on screen 70.FIG. 7 shows rotatable blade 62d meeting screen 70 to return wasteretained by screen 70 to cutting surface 68.

Disinfecting stage 22 comprises a disinfectant reaction chamber 76 whichis integral with auger 74. Auger 74 is inclined upward away from augerinlet 78 to enable precise control of the waste residence time inreaction chamber 76 and to facilitate dewatering as described hereafter.The inclination angle of auger 74 is defined as φ. For a waste streamcomposed mostly of soft material, such as medical waste, φ is selectedbetween about 10° and 20° and preferably about 15° . This aids indewatering, without promoting clogging. A lesser angle results in lessdewatering capability, while a greater angle appreciably increases thetendency to clog. Reaction chamber 76 is sufficiently sized to hold thethroughput of system 10 for a residence time which enables disinfectionof the waste before discharge from system 10. Auger 74 has a screw 80extending axially the entire length of auger 74 which is rotatablymounted therein to carry waste from auger inlet 78 to a waste soliddischarge port 86 at the upper end of auger 74.

Dewatering stage 24 is likewise integral with auger 74 and comprises aconical flow restriction 90 at solid disinfected waste discharge port86. A portion of liquid medium exits auger 74 under gravity through port82 to collection tank 56 in fluid communication with port 82. Aperforated plate 88 is provided at port 82 having a plurality ofperforations 89, each significantly smaller than the mesh size of screen70, and preferably about 1/8 inch, to prevent substantial quantities ofwaste from exiting auger 74 thereat. However, the primary function ofport 82 is to enable fluid intrusion into auger 74 as will be shown.

The conical flow restriction 90 imposes a pressure on the waste materialwhich compacts the material and removes the bulk of liquid medium fromthe waste before it exits system 10. The auger screw 80 terminatesslightly beyond the entrance to the conical restriction 90. In oneembodiment the constriction is a conical nozzle 90 having a fixedopening at the end of waste discharge port 86. The angle of the conicalrestriction 90 is selected according to the content of the waste stream.For a waste stream composed mostly of soft material, such as medicalwaste, the angle is between 15 and 20 degrees, and preferably about 18degrees. This ensures sufficient compaction of waste material to achievedewatering, without clogging the flow path. A lesser angle wouldsignificantly detract from the dewatering ability, while a greater anglewould significantly increase the tendency to clog. In anotherembodiment, FIG. 4 shows an adjustable nozzle comprising a pair of doors92a, 92b, the lower door having a pneumatically biased hinge 93 torender the size of opening 91 pressure responsive. In any case, therestriction applies a compacting force to the disinfected waste beforethe waste exits the system 10.

Liquid medium driven from the disinfected waste by the compacting forceexits auger 74 through perforations 94 in auger housing screen 96.Perforations 94 are sized small enough to restrict the solid waste fromthe liquid stream. A sleeve 98 around screen 96 at perforations 94channels the liquid medium into a recycle line 100 which is in fluidcommunication with the mixing tank 156 through recycle inlet line 144.Before being recycled to the mixing tank 156, as shown in FIG. 9, theliquid is passed through a cyclone separator 140 by means of a pump (notshown). The liquid cycles through the separator 140 to exit into therecycle inlet line 144, after the separation of heavy fines 146 or othermaterials which fall to the bottom of the separator 140. Periodically, avalve 150 is opened to flush the heavy fines 146 out the outlet 148 ofthe separator 140 and back onto the waste material on the auger 74.

Collection tank 56 has two chambers 104, 106 in fluid communication withone another, but separated by a weir 108. Port 82 of auger 74 issubmerged in primary chamber 104. Secondary chamber 106 receives theoverflow of primary chamber 104 and has a recycle outlet port 110connected to recycle line 54. Heater elements 112, 114 are submerged inprimary and secondary chambers 104, 106 respectively for heating theliquid medium as necessary. The collection tank 56 and the mixing tank156 can be combined as one tank without departing from the spirit of theinvention.

FIG. 5 is a schematic for process control of system 10 which is providedby automated control unit 120 in electrical communication with auger 74,heaters 112, 114, recycle pump 48 and door 92b. If a dewatering cone isused, instead of the adjustable door 92b, this connection to the controlunit is deleted. Control unit 120 accordingly regulates the speed ofauger screw 80, the heat output of heaters 112, 114, the liquid mediumrecycle rate of pump 48 and the compaction force applied by door 92b tothe waste at solid waste discharge port 86. These parameters areregulated in response to the primary input parameters to unit 120 whichare the ClO₂ concentration and the temperature of the liquid medium intank 56. ClO₂ concentration data is provided to unit 120 by means of aconventional air stripper 122 in tank 56 and ClO₂ gas analyzer 124.Temperature data is provided to unit 120 from a conventional temperaturesensor 126.

Method of Operation

With cross-reference to the drawings, operation of system 10 in acontinuous mode may be seen. System 10 is particularly suited to thetreatment of infectious wastes generated by hospitals and other medicalfacilities. Such wastes are primarily solid wastes consisting ofplastic, paper, fabric, glass, and metal and embody a broad range ofmedical items including syringes, bottles, tubes, dressings, and thelike. "Waste treatment" as the term is used herein constitutesfragmenting of the waste to a relatively small granular particle sizeand disinfecting the waste to render it substantially innocuous andsuitable for ordinary landfilling.

The infectious waste is fed through inlet opening 28 into system 10 inany form. In a preferred embodiment, however, the waste is stored in asealed compartmentalized plastic bag 128 which is then fed throughopening 28 into system 10 in its entirety. Waste bag 128 has a primarycompartment 130 containing the infectious waste, and the bag can haveother prefilled and sealed compartments 132, 134 containing disinfectantchemicals or other process additives, if required. Additives may includedyes, defoamers, or surfactants.

The waste is inserted through inlet opening 28 into the top offragmenting chamber 30 by an operator. The waste drops under the forceof gravity from opening 28 down into opposingly rotating shreddingblades 31a,b, 32a,b, 33a,b of shredding stage 14. The shredding bladesdestroy waste bag 128, spilling the waste and additives into chamber 30where they are commingled to form a waste mixture. The shredding bladesalso break up the frangible waste to a small particle size. Wettingstage 16 operates simultaneously with stage 14, whereby the disinfectantjets wet the waste mixture with a stream of a liquid disinfectant. Theliquid disinfectant is pumped to the jets from lines 54 and 62 connectedto liquid medium collection tank 56 and mixing tank 156. With efficientoperation of dewatering stage 24, the bulk of liquid medium in system 10is recycled. The liquid disinfectant may be within a temperature rangebetween about 0° C. and 100° C. and preferably between about 5° C. and70° C. The liquid medium has more preferably been preheated aboveambient temperature.

The liquid disinfectant uniformly contacts the falling waste mixture toform a wet mash. The mash falls through metal segregating stage 58 wheremetals are removed and continues falling down into granulating stage 18where the rotating blades and the stationary blades break up the alreadysmall particle size frangible waste into yet a smaller granular particlesize which is preferably slightly less than 1/4 inch. The granulatingblades also fragment any fibrous material which has not been previouslyfragmented by the shredding blades, to about the same smaller granularparticle size as the frangible material. The granulating blades alsomore fully mix the mash. Thus, the solids in the resulting mash ofgranulating stage 18 are preferably fully wetted by the disinfectantsolution and the bulk of the solids preferably have a smaller granularparticle size which is slightly less than about 1/4 inch. The liquidcontent of the mash is typically on the order of about 60% by weight.

Upon exiting granulating stage 18, the mash drops onto screen 70 whichfunctions in cooperation with the granulating stage 18 to allow thesmaller granular particle size waste to fall through it intodisinfectant reaction chamber 76 while retaining any waste ingranulating stage 18 which has not been sufficiently fragmented. Wastewhich is retained by screen 70 is scooped up by the rotating granulatingblades rotating against screen 70, and returned to cutting surface 68for additional particle size reduction until it is sufficiently small topass through screen 70.

Inlet port 78 receives the waste mash from screening stage 20 anddirects the mash to reaction chamber 76 integral with auger 74. Thedisinfectant solution collected in primary chamber 104 contacts the mashat lower end 84 of auger 74. Auger screw 80 turns continuously towithdraw the mash from lower end 84 at angle φ up the auger incline tosolid waste discharge port 86 at a controlled rate which allows asufficient residence time of the mash in reaction chamber 76. Asufficient residence time is typically on the order of less than about 5minutes and preferably on the order of about 3 minutes. Auger screw 80also maintains perforated screen 96 free of waste so that the liquidmedium may exit the auger to be recycled. The disinfected and dewateredwaste exiting system 10 typically has a liquids content of about 20% byweight in contrast to a liquids content in the mash of about 60% byweight.

The bulk waste volume of the exit waste is on the order of about 15% ofthe inlet waste. Most of the liquid medium is removed from the waste asthe result of compaction caused by fixed nozzle 90 or pressureresponsive nozzle 92a,b positioned at waste discharge port 86. Theliquid medium exits auger 74 through perforations 94 and is collected intank 156 for recycling to wetting stage 16 via line 162. Alternatively,collection can be in tank 56. The dual-chamber weir arrangement of tank56 enables collection of fines in primary chamber 104 for periodicremoval.

Process control for system 10 is provided by control unit 120. Thedecontamination level, i.e., level of kill, attainable in system 10 is afunction of several interrelated operating parameters including liquidmedium flow parameters and auger and heater operating parameters asshown in FIG. 5. Nevertheless, as is shown below, an operational modelof system 10 can be developed as a function of a limited number of keyparameters, which are level of kill, disinfectant concentration andtemperature.

Accordingly, process control can be effected by selecting a desiredlevel of kill, i.e., target kill, and adjusting the disinfectantconcentration and disinfectant solution temperature as a function of theoperating parameters to meet the preselected target kill. For example,in theory, a target kill of 6 decades (10⁶ organisms/ml) is achievedwithin about three minutes for a typical infectious medical waste usinga chlorine dioxide solution at a concentration of 30 ppm and atemperature of 50° C. In practice, however, the process is controlled byadjusting only temperature while monitoring variations in thedisinfectant concentration as a baseline for temperature adjustment.Temperature is selected as the independent variable and disinfectantconcentration as the dependent variable for the practical reason thatthe ability to independently adjust disinfectant concentration issomewhat limited when a fixed amount of precursor is employed, while itis relatively easy to adjust solution temperature via heaters 112, 114.Added amounts of precursors can be provided for process startup, or inthe event of process upsets.

The operational model of system 10 recognizes the functionalrelationship between solution temperature and concentration of thedisinfectant, chlorine dioxide, at a given level of kill n. The model isrepresented by the equation:

    [ClO.sub.2 ]=a.sub.n e.sup.-k.sbsp.n.sup.T                 (1)

wherein

[ClO₂ ]=chlorine dioxide concentration,

T=temperature, and

a_(n), k_(n) =empirically determined constants for kill_(n).

FIG. 6 generally depicts the shape of the curve for equation (1). Eachpoint on the curve defines values of [ClO₂ ] and T at which kill_(n) canbe achieved. Accordingly, process control is more specificallyimplemented by preselecting the target kill, empirically determining themodel constants at the target kill to define a curve, and adjusting theactual values of [ClO₂ ] and T to lie on the target kill curve.

FIG. 6 shows a typical start-up scenario for system 10. The treatmentsolution is initially at point A which is inside the required curve forthe target kill. Since it is desirable to operate on the curve,automated process control 120 consequently raises the temperature of thesolution in tanks 56, 156 toward point B which corresponds to the samechlorine dioxide concentration as point A, but at a higher temperature.Raising the temperature of the solution, however, increases the rate ofchlorine dioxide formation, thereby increasing the chlorine dioxideconcentration of the solution to a value designated by C on the verticalaxis. Thus, as point B is approached, control unit 120 calculates thatthe required temperature on the curve has fallen. The dashed line showsthe iterative equilibration procedure followed by control unit 120whereby an operating point designated by D is ultimately attained.Operation is preferably maintained along or above the locus of pointsmaking up the curve which includes point D.

Chlorine dioxide concentration in tank 56 is continuously monitored bymeans of air stripper 122 and gas analyzer 124 to enable control unit120 to determine whether the requirements of the disinfectant solutionhave changed. For example, if a relatively "dirty" waste is fed tosystem 10, the amount of ClO₂ consumed increases, reducing the ClO₂concentration in the solution. Accordingly, control unit 120 mustiteratively increase the temperature of the solution in the mannerrecited above to return operation of system 10 to the curve. If arelatively "clean" waste is fed to system 10, the ClO₂ concentrationincreases, correspondingly reducing the temperature requirement. Thus,control unit 120 decreases the temperature of the solution. It ispreferable to preselect a target kill exceeding a minimum acceptablelevel of kill so that adequate decontamination of the waste is achievedeven when operation falls somewhat below the curve. It has generallybeen found that within the presently prescribed temperature range aminimum ClO₂ concentration in the treatment solution to achieve anacceptable level of kill is about 10 ppm up to the requiredconcentration and preferably about 12 ppm up to the requiredconcentration.

FIG. 10 shows a typical operating curve which has been found to beeffective for practical operation of the apparatus of the presentinvention. The recommended solution temperature in degrees C is plottedversus the measured chlorine dioxide concentration in ppm. Two curvesare shown, reflecting an upper recommended limit and a lower recommendedlimit, with the area between the curves representing the normaloperating area, or system control band. Approximately 3 degrees belowthe lower recommended limit is an alarm curve. It can be seen from FIG.10 that the recommended control band for a chlorine dioxideconcentration of 30 ppm is between approximately 21 degrees andapproximately 41 degrees, with an alarm point at approximately 18degrees. In fact, the upper recommended temperature limit is 20 degreesabove the lower limit for any given concentration. The lower recommendedlimit curve in FIG. 10 corresponds to the curve shown in FIG. 6.

As noted in the preferred embodiment above, starting quantities of thechlorite salt and acid are fixed. As such, they are preferably providedin stoichiometric excess of quantities necessary to produce the requiredchlorine dioxide concentrations. Thus, adequate concentrations of liquidprecursors will be available in solution for chlorine dioxideproduction, with equilibrium between the precursors and the reactionproducts being controlled by tank temperature. A significant fraction ofthe chlorine dioxide is consumed by reaction with the infectious wasteconstituents or diffuses out of solution. By way of example, a typicalrelative starting concentration of precursors, solvent and waste whichwill provide a desired chlorine dioxide concentration, is on the orderof 4.6 g/l sodium chlorite per 3.3 g/l citric acid per 12 kg of solidwaste.

While the particular Improved Multi-Stage Infectious Waste TreatmentSystem as herein shown and disclosed in detail is fully capable ofobtaining the objects and providing the advantages herein before stated,it is to be understood that it is merely illustrative of the presentlypreferred embodiments of the invention and that no limitations areintended to the details of construction or design herein shown otherthan as described in the appended claims.

We claim:
 1. An apparatus for treating infectious waste, comprising:aninlet opening sized to receive an infectious waste; a shredding stagepositioned to receive the waste from the inlet opening, for shreddingthe waste to a first, small particle size; a wetting stage positioned tomix and inject a disinfectant fluid containing a volatile disinfectantinto the shredded waste downstream of said shredding stage, saidinjection occurring substantially at the time of mixing of said volatiledisinfectant, to prevent disassociation of said volatile disinfectant; agranulating stage having rotatable granulator blades positioned toreceive the shredded waste from said wetting stage, to granulate theshredded waste to a second, smaller granular particle size and togranulate any strips present in the shredded waste material; adisinfecting stage positioned to receive the granulated waste from saidgranulating stage, said disinfecting stage comprising a reaction chambersized to retain the granulated waste for a selected residence time; anauger conveyor positioned to transport the granulated waste from saidreaction chamber to an exit opening; a dewatering stage comprising aconical flow restriction in the waste flowpath between said augerconveyor and said exit opening for removing said disinfectant fluid fromsaid waste; at least one heater for heating said disinfectant fluid; anda control system for sensing the temperature of said disinfectant fluid,for sensing the concentration of said volatile disinfectant in saiddisinfectant fluid, for selecting an optimum disinfectant temperaturecorresponding to said disinfectant concentration, and for controllingsaid at least one heater to maintain said temperature of saiddisinfectant in a control band above said selected optimum temperature,to achieve a desired organism kill rate.
 2. An apparatus for treatinginfectious waste as recited in claim 1, further comprising:a collectiontank for collecting said disinfectant fluid; an air stripper forstripping gases from said disinfectant fluid in said collection tank;and a gas analyzer for analyzing said gases stripped from saidcollection tank to determine said concentration of said volatiledisinfectant.
 3. An apparatus for treating infectious waste as recitedin claim 2, wherein said at least one heater is located in saidcollection tank.
 4. An apparatus for treating infectious waste asrecited in claim 1, wherein said control band has a band width ofapproximately 20 degrees Celsius.
 5. An apparatus for treatinginfectious waste, comprising:an inlet opening sized to receive aninfectious waste; a shredding stage positioned to receive the waste fromthe inlet opening, for shredding the waste to a first, small particlesize; a wetting stage positioned to mix and inject a disinfectant fluidcontaining a volatile disinfectant into the shredded waste downstream ofsaid shredding stage, said injection occurring substantially at the timeof mixing of said volatile disinfectant, to prevent disassociation ofsaid volatile disinfectant; a granulating stage having rotatablegranulator blades positioned to receive the shredded waste from saidwetting stage, to granulate the shredded waste to a second, smallergranular particle size and to granulate any strips present in theshredded waste material; a disinfecting stage positioned to receive thegranulated waste from said granulating stage, said disinfecting stagecomprising a reaction chamber sized to retain the granulated waste for aselected residence time; an auger conveyor positioned to transport thegranulated waste from said reaction chamber to an exit opening; adewatering stage comprising a conical flow restriction in the wasteflowpath between said auger conveyor and said exit opening, for removingsaid disinfectant fluid from said waste; a collection tank forcollecting said disinfectant fluid; a temperature sensor in saidcollection tank for sensing the temperature of said disinfectant fluid;at least one heater located in said collection tank for heating saiddisinfectant fluid; an air stripper for stripping gases from saiddisinfectant fluid in said collection tank; a gas analyzer for analyzingsaid gases stripped from said collection tank to determine aconcentration of said volatile disinfectant in said disinfectant fluid;and a control unit for receiving the temperature of said disinfectantfluid from said temperature sensor, for receiving the concentration ofsaid volatile disinfectant from said gas analyzer, for selecting anoptimum disinfectant temperature corresponding to said disinfectantconcentration, and for controlling said at least one heater to maintainsaid temperature of said disinfectant fluid in a control band above saidselected optimum temperature, to achieve a desired organism kill rate.6. An apparatus for treating infectious waste as recited in claim 5,wherein said control band has a band width of approximately 20 degreesCelsius.